This invention relates to a carrier wave recovery apparatus for deriving a reference carrier wave in a demodulator adapted to demodulate a PSK modulated wave based on synchronous detection.
Various proposals have been made for the carrier wave recovery apparatus for restoring a carrier wave from a carrier-suppressed input modulated-wave. Among those proposals, the approach based on signal-processing in the base band region is advantageous over others based on signal-processing in the carrier wave region, in that the circuit for the signal processing is easy to design in a relatively low frequency range, and that the restored carrier wave can be automatically brought to an optimum phase. The carrier wave recovery system of the base band processing type is known as a decision feedback system. This system is introduced in the article entitled "Phase-Locked-Loop Detection of Binary PSK Signals Utilizing Decision Feedback" by F. C. Natali and W. J. Walbesser, published in "IEEE Transactions on Aerospace and Electronic Systems" (Vol. AES-5, No. 1, January 1979, PP. 83-90). An example of this system is disclosed in the article entitled "Mirowave QPSK Demodulator Techniques at the Receiver Front End" by C. L. Cuccia, published in pp. 40-42 of "1977 IEE-MTT-S INTERNATIONAL MICROWAVE SYMPOSIUM". According to the circuit disclosed in FIG. 2 in the latter article, the output of a voltage controlled oscillator (VCO) serving as a carrier wave source is supplied directly and through a 90% phase shifter respectively to first and second phase detectors as reference carrier waves for synchronous detection of an input modulated-wave; then the input modulated wave is coherently detected; these coherently detected outputs are applied to first and second comparators for determining the polarity of the output voltages of the first and second phase detectors and providing a predetermined output voltage of the detected polarity or the polarity opposed to the detected one; the product of the second (or first) coherently detected output and the second (or first) comparator output is multiplied in a first (or second) analog multiplier; then the difference output between the first and second analog multiplier outputs is derived from a subtractor; and the VCO is controlled by a voltage representing the above difference for deriving a reference carrier wave.
However, this prior art circuit requires a broader dynamic range for the multipliers, since the output voltage level of two multipliers for bringing VCO input to 0 volt is high. The multipliers of a wide output voltage range are complicated in construction and costly to manufacture.